This invention relates, in general, to surface acoustic wave (SAW) devices, and more specifically, to acoustic charge transport (ACT) devices.
ACT is a relatively new technology relating to SAW devices. ACTs can perform many functions such as delay and transversal filtering, and are formed on Galium Arsenide (GaAs) substrates to take advantage of the high processing speed of GaAs. The ACT devices may be used in radios employing SAW filters, amplifiers which compensate for SAW filter insertion loss, and MMIC switches for changing filters.
As explained in U.S. Pat. No. 4,633,285, issued Dec. 30, 1986 to Billy Jo Hunsinger and assigned to University of Illinois, an ACT comprises a buried channel formed by placing a piezoelectric semiconductor material between confining layers of GaAs. A SAW is then generated within the piezoelectric layer. As the SAW propagates through the piezoelectric layer, an input signal is injected into the wells of the SAW from which majority carriers have been depleted. The signal is carried by the SAW below electrodes which are capacitively coupled to the piezoelectric layer. The electrodes can extract the magnitude of the charge in each SAW well without actually extracting charge from the signal. This process is termed non-destructive sensing since the capacitively extracted charge magnitude does not deplete the charge in the SAW well.
A specific use of ACT filters is digitally programmable transversal filters, or DPTF. One such DPTF is described in AN ACOUSTIC CHARGE TRANSPORT DIGITALLY PROGRAMMABLE TRANSVERSAL FILTER, by R. Miller, C. Ricci, and R. Kansy, IEEE JOURNAL OF SOLID-STATE CIRCUITS, Vol. 24, No. 6, December 1989. The taps of the ACT described within this article are each programmable to give a variable weighting for the combinations of taps. Significant relative space is required on the GaAs structure to support the circuitry necessary to separately program each tap. Furthermore, programming of the individual taps for each new input signal requires valuable time.